This paper highlights results from a fundamental study into the fate of resin acids during biological treatment. Pulp mills in Canada rely on biological treatment systems for the removal of resin acids that are released from wood during pulping and bleaching. These are priority contaminants for the pulping industry since they have been associated with events of toxicity breakthrough. Although tighter mill control has helped to minimise the frequency of these events, it would be useful to have an understanding of the basic limitations of biological systems as the last line of defence for limiting resin acid discharges. The dependence of physico-chemical and biological phenomena on pH will influence the chances for successful biological removal of resin acids. Changes in pH within the typical range used for biological treatment significantly alter the bioavailability of resin acids and the ecology of the microorganisms responsible for their degradation. Changes in resin acid loading during continuous treatment also affect the microbial ecology. Further, the capacity of a treatment system to degrade resin acids is a function of the contaminant loading. Time delays for microbial community acclimation in response to a shift-up in loading are significantly long and are also a function of pH. The capacity to remove resin acids is readily gained and lost in a biological system. Hence the bioreactor operating conditions in conjunction with the period and amplitude of loading fluctuations can impact on the extent of biological removal for resin acids. Consequently, biological systems can be severely compromised by influent transient loading when it comes to the removal of specific contaminants like resin acids.

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